154 



HORN LOUD SPEAKERS 



From the voice coil electrical circuit, Fig. 8.1, the total electrical impedance 

 Zet in ohms at e is 



Zet = rsD + Teg + jwL + Zem 



where Ted = voice coil resistance, in ohms, 



rsG = amplifier output resistance, in ohms, 

 L = inductance of the voice coil, in henries, and 

 e = amplifier open circuit voltage, in volts. 

 Equation 8.16 shows that the throat acoustic resistance may vary over 

 wide limits without introducing large variations in the power output. As 



a specific example. Fig. 8.5 

 shows the power output as a 

 function of the frequencv for a 

 horn, having all dimensions two 

 and one half times that of Fig. 

 S.6B and driven by a vacuum 

 tube having the constants indi- 

 cated by the caption of Fig. 8.5. 

 500 Although the variation in acous- 

 tic resistance is 6 to 1 the varia- 

 FiG. 8.5. Power output characteristic of the horn ^^^^ j. ^^^ ^^ jg ^^^v 2 db. 



(Fig. 5.65 with all dimensions multiplied by t^ rr.? -nrc r i tt- ■ ^ •; 



Ih) coupled to a IQi-inch diameter, 10-gram E. 1 he h^ect oj the l^OtCe Lotl 

 diaphragm driven by a 5-gram aluminum voice Temperature Upon the Efficiency ^^. 

 coil in a field of 20 000 gausses. Damped _ -j^j^^ ^^^^^ ^f ^y^^ temperature 

 resistance of voice coil, 20 ohms. Impedance r \ • m u rn 



ofvacuum tube through a transformer, 35 ohms, ot the VOlce COll upon the effi- 

 ciency is usually ignored in con- 

 siderations of the performance of a loud speaker. In high power loud 

 speakers, where the temperature of the voice coil becomes quite high, 

 considerable loss in efficiency may result as will be shown in the discussion 

 which follows. 



The efficiency of a loud speaker, when the temperature correction is 

 added, may be expressed 



1--4 



D 

 O 



-8 



100 200 



FREQUENCY 



M = 



Tem 



rEmiy + oit) + rEM 



8.17 



where rEm = damped resistance of the voice coil at 0° Centigrade, 



a = temperature coefficient of resistance, 0.00423 for aluminum 

 and 0.00427 for copper. 



15 Olson, H. F., RCJ Review, Vol. 1, No. 4, p. 68, 1937. 



